1. Field of the Invention
The present invention relates to a method of manufacturing thin film diodes provided in a liquid crystal display, and more particularly to a method of fabricating thin film diodes which are nonlinear resistance elements serving as switching elements against a multitude of pixels arranged in a matrix form on a display face of the liquid crystal display.
2. Description of the Related Art
Along with an advance in commercial application of liquid crystal displays, liquid crystal displays (hereinafter referred to as "LCD") of an active matrix type, capable of displaying high quality images, are about to occupy a dominant position in the market.
The active matrix LCD described above is provided with nonlinear resistance elements comprising thin film transistors (TFTs) or thin film diodes (TFDs) of a (metal-insulator-metal) laminated structure, composed of metal-anodic oxidation film-metal layers or metal-anodic oxidation film-transparent and electrically conductive film layers, as switching elements against respective pixel electrodes arranged in a matrix form on the display face of the LCD.
With the LCD provided with the thin film diodes serving as switching elements, images are written by switching the thin film diodes on and off so as to apply voltages to the respective pixel electrodes connected in series to the thin film diodes, taking advantage of a nonlinear current-voltage characteristic of the thin film diodes.
An example of the construction of a liquid crystal display provided with such thin film diodes as described above is shown in FIG. 34.
FIG. 34 is a perspective view of the LCD of the active matrix type for display in color showing schematically the construction thereof.
The LCD comprises a first substrate 1 and a second substrate 2 which are both made of glass and disposed in parallel so as to face each other, forming a gap therebetween, and liquid crystals 3 are filled in the gap. A multitude of pixel electrodes 4 composed of transparent and electrically conductive films are formed in a matrix configuration on the upper surface of the first substrate 1, and are connected via respective thin film diodes 5 with signal electrodes 6 extended along rows.
On the other hand, color filters 7 for R (red), G (green), and B (blue), respectively, and facing electrodes 8 disposed opposite to the pixel electrodes 4 and extended along columns perpendicular to the signal electrodes 6 are provided on the underside surface of the second substrate 2.
Further, polarizers 9 and 10 are disposed on the external surfaces of the first substrate 1 and the second substrate 2, respectively, and white light is irradiated from under the first substrate 1 as shown by the arrows in the figure.
Then, voltages are applied between the respective pixel electrodes 4 and the respective facing electrodes 8 selectively via the respective signal electrodes 6 and the respective thin film diodes 5, thereby creating electric fields, by which data are written into the liquid crystals 3 sandwiched between both electrodes as described in the foregoing so that images are displayed by controlling the transmission of irradiated light.
An LCD for displaying images in monochromatic mode can be manufactured by dispensing with the color filters 7.
The constitution of the thin film diode 5 used in the LCD is described hereafter with reference to FIGS. 35 and 36. FIG. 35 is a plan view showing the pixel electrode 4 constituting one pixel, the thin film diode 5, and the signal electrode 6, which are disposed on the first substrate 1 as shown in FIG. 34, and FIG. 36 is an enlarged sectional view taken along line C--C in FIG. 35.
The thin film diode 5 comprises a lower electrode 13 connected with the signal electrode 6, an anodic oxidation film 15 formed on the surface of the lower electrode 13 as shown in FIG. 36, and an upper electrode 17 formed in such a way as to overlie the lower electrode 13 via the anodic oxidation film 15 and connected with the pixel electrode 4.
A conventional method of fabricating such a thin film diode as described above has been disclosed in, for example, "IEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-28 NO. 6 JUNE 1981 736-739". The aforesaid method is briefly described with reference to FIGS. 35 and 36.
A lower electrode material made of tantalum nitride (TaNx) is first applied to the entire surface of a substrate 1 made of glass in an atmosphere of a mixture of an argon gas and a nitrogen gas by use of the reactive sputtering method.
Thereafter, a photoresist is formed on the entire surface of the lower electrode material film by use of a spin coater, and patterning on a photoresist (not shown) is provided in the form of the lower electrode 13 by means of exposure and development treatments applied to the photoresist using a predetermined photo mask. Formation of the photoresist on the entire surface combined with the exposure and development treatments using the photo mask is hereafter referred to as a photolithographic treatment.
Then, the lower electrode 13 is formed by etching the lower electrode material film, that is, the tantalum nitride, using the patterned photoresist as an etching mask. As shown in FIG. 35, the lower electrode 13 is formed in a plane pattern in such a way as to protrude from the signal electrode 6 perpendicularly.
Thereafter, by applying an anodic oxidation treatment to the lower electrode 13, an anodic oxidation film 15 composed of a tantalum oxide (Ta.sub.2 O.sub.5) film is formed on the surface of the lower electrode 13 as shown in FIG. 36. The anodic oxidation treatment is carried out by applying a voltage at 36 V using, for example, an aqueous solution containing 0.1 wt % of citric acid as anodic oxidation solution.
Subsequently, by applying the vacuum evaporation method, an upper electrode material film made of nickel-chrome gold composed of a transparent and electrically conductive film is formed.
Thereafter, patterning is made on a photoresist film (not shown) by applying the photolithographic treatment thereto, and then using the patterned photoresist film as an etching mask, the upper electrode material film is etched, thus forming the upper electrode 17.
As shown in FIG. 35, the upper electrode 17 is formed in a plane pattern such that an opening is defined in a part of the region for the pixel electrode 4, and the upper electrode 17 overlies the lower electrode 13.
The method of fabricating the thin film diode, described in the foregoing has an advantage in that the number of processing steps is reduced and, particularly, patterning on the photoresist film needs to be done only twice. By using the tantalum nitride film as the material of the lower electrode 13, nonlinear current-voltage characteristics of the thin film diodes can be increased, thereby improving switching characteristics.
However, LCDs provided with the thin film diodes fabricated by this method pose a problem of an after-image phenomenon occurring every time when the whole display is switched in the course of driving the LCDs.
Referring to the diagram shown in FIG. 37, the after-image phenomenon is described in detail hereafter. It is to be pointed out that the LCD in this case is of normally white display mode and provided with two polarizers 9 and 10 as shown in FIG. 34 which are disposed such that light is allowed to be transmitted when no voltage is applied. In FIG. 37, the ordinate and abscissa of the diagram indicate relative transmissivity (%) and time (minutes), respectively.
The figure shows variation in relative transmissivity of light when a voltage applied to a pixel at random is varied at an interval of 5 minutes. More specifically, a voltage for display at 50% of transmissivity is applied first for 5 minutes (half-tone displaying period: T1), then a different voltage for display at 10% of transmissivity is applied for the next 5 minutes (black displaying period: T2), and further the same voltage as applied for the first half-tone displaying period, that is, T1, is applied for yet another 5 minutes (half-tone displaying period: T3).
The after-image phenomenon is a phenomenon wherein a difference (.DELTA.T) in transmissivity between the first half-tone displaying period T1 and the next half-tone displaying period T3 occurs at the outset of the period T3 although voltages applied for respective periods remain the same. With an LCD using the thin film diodes fabricated by the conventional method described above, the difference (.DELTA.T) in transmissivity was found to be about 5%.
The occurrence of the after-image phenomenon described above results in the display of images with their contents different from those of the originally intended images.
It means that the after-image phenomenon, also called the image-sticking phenomenon, causes the quality of images displayed by the LCD to be degraded considerably, posing a serious problem in commercial applications of the LCD.